The Big Picture: Circulation and Gas Exchange


  • Direct exchange between every cell and the environment is not possible in all animals
    • special structures for exchange & transport needed


  • Respiratory systems:


  • Circulatory systems:


  • Internal transport and gas exchange functionally related

Molecule trading


  • Small molecules can move between cells and their surroundings by diffusion


  • Diffusion is only efficient over small distances


  • Molecule trading in animals occurs for every cell!
    • gain O2 and nutrients
    • release CO2 and waste products


  • Simple body plans:
  • Complex body plans:

Animals with gastrovascular cavities


  • Many animals have lots of cells in contact with environment
    • hydras, jellyfish, flatworms


  • Central gastrovascular cavity functions to distribute substances
    • opening at one end connects cavity to water
    • allows fluid to also bath inner tissues
    • diffusion distance with environment is very short


  • Gastrovascular cavity can be highly branched
    • extends into tentacles of cnidarians
    • covers high surface area within flatworms

Animals with circulatory systems (most bilateral animals)


  • Circulatory systems have 3 components


  • Fluid transport, via vessels, connects cytoplasm of cells to the gas exchange organs


  • Mammals → O2 diffuses over just 2 cell layers in lungs to reach blood


  • Molecule trading with circulatory fluid often works like a circuit

Open and closed circulatory systems (hemolymph vs blood)

Vertebrate Circulatory systems


  • Cardiovascular system: blood → heart → vessels
    • length of vessels can be staggering


  • 3 main types of vessels
  1. Arteries:
  2. Veins:
  3. Capillaries:


  • Hearts of vertebrates have 2+ muscular chambers
    • atria:
    • ventricles:
  • Number of chambers related to form and function

Single vs Double circulation in vertebrates

Evolutionary variation: 3 chambered heart



  • Amphibians and reptile hearts have 1 ventricle
    • still use double circulation


  • Do not always fill their lungs
    • pass long periods without gas exchange
    • or use another tissue (skin)


  • Example: frogs use skin when underwater
  • Example: crocodiles shunt blood from lungs when underwater

Mammalian Circulation (1st circuit)



  • The heart contracts and relaxes in a rhythmic cycle called the cardiac cycle


  • Right ventricle pumps blood to the lungs via arteries


  • Blood flows through capillaries into lungs and loads O2 and unloads CO2


  • O2 rich blood returns from lungs via veins to the left atrium

Mammalian Circulation (2nd circuit)


  • Blood flows to left ventricle and pumped to tissues via arteries into capillaries
    • First branches supply the heart muscle
    • Further branches lead to organs and limbs


  • O2 diffuses from blood to tissues, and CO2 diffuses from tissues to blood


  • Capillaries rejoin conveying blood to veins
    • O2 poor blood from the head, neck, and forelimbs is emptied into right atrium

Blood flow is reduced in capillaries



  • Physical laws govern movement of fluids through pipes
    • affect blood flow and pressure


  • Blood flows from areas of higher pressure to low
    • narrow diameter capillaries decrease pressure


  • Blood flow is slowest in the capillaries
    • over large cross-sectional area
    • necessary for efficient gas exchange

Gas exchange over respiratory surfaces




  • Gas exchange = uptake of O2 and release of CO2
    • O2 is abundant in the air (21%)
    • fairly easily to ‘breath’


  • Water is more demanding for gas exchange
    • dissolved O2 is variable but always less than air


  • Efficient surfaces for gas exchange have evolved across animals that live in water and on land
    • form is always going to include high surface area

Respiratory Surfaces


  • Differs across animal phylogeny
    • sponges, cnidaria, flatworm → cells directly
    • earthworms, amphibians → skin


  • Other animals lack enough surface area
    • to many inner cells to supply with O2


  • Respiratory organs evolve
    • Gills → aquatic animals
    • Trachea → insects
    • Lungs → reptiles and mammals

Gills in fish

Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. Within the gill filaments, capillary blood flows in the opposite direction to the water

Trachea in insects

Trachea are a series of tubes that open to the air and connect internally to the circulatory system

Lungs in mammals, reptiles and birds

Elastic sacs with branching passages into which air is drawn, so that oxygen can pass into the blood and carbon dioxide be removed

Coordination of circulation and gas exchange



  • Animals need to exchange large amounts of O2 and CO2
    • to meet metabolic needs of all cells


  • During inhalation, fresh air fills in lungs
    • molecule trading occurs via diffusion


  • Blood is then pumped through circuit
    • O2 carried in pigments (metal bound to protein)
    • CO2 picked up from body cells and returned to sites of exchange with environment